It is conventionally known that as a means for performing rapid heating, induction heating is effective. However, since a heating method using induction heating utilizes electromagnetic induction, when a plurality of heating coils each having a power control means (for example, inverter) are arranged adjacently and are operated, mutual induction occurs in each of the heating coils.
In order to avoid the effect of the mutual induction and properly operates the inverter which supplies electricity to each of the heating coils, it is necessary to equalize the frequency of each inverter and synchronize its current (see patent document 1).
The reason why the frequency is equalized is that when the mutual induction of different frequencies occurs, an inverter current and an inverter voltage have a distorted waveform, it is impossible to properly operate the inverter. The reason why the current is synchronized is that when it is assumed that a mutual induction voltage is jωM·I2·(cos θ+j sin θ), if the coil current is synchronized, θ=0, and the mutual induction voltage is jωM·I2, with the result that only the reactance component of a mutual induction impedance is left. On the other hand, when the coil current is not synchronized, based on the phase difference of θ, the mutual induction voltage is indicated as jωM·I2·cos θ−ωM·I2·sin θ, and the resistance component of the mutual induction impedance appears. Hence, power sharing between the inverters is changed by the mutual induction, and this affects power control on the inverters (co is an angular frequency, M is the mutual inductance caused by the mutual induction between the heating coils arranged adjacently and I2 is the current that is supplied to the heating coils arranged adjacently).
In normal induction heating, a resonance sharpness is 3 to 10, and a coil-to-coil coupling coefficient k is about 0.2. In a series inverter, a coil voltage 10 times as large as an inverter voltage is produced. A voltage about 0.2 times as large as the coil voltage becomes a mutual induction voltage. When θ=30 degrees, the value of the effective part of the mutual induction voltage, that is, the resistance component of the mutual induction impedance, is equal to the inverter voltage, with the result that this significantly affects the power control on the inverter. In order to avoid this effect, it is necessary to perform current synchronization control.
However, even when the current synchronization control is performed, the mutual induction voltage of an reactive part, that is, the voltage caused by the reactance component of the mutual induction impedance is left. This mutual induction voltage is varied by a variation in the coil current on the side that gives the effect. Here, an impedance and a phase caused by mutual induction between a resonant capacitor of a resonant circuit and a self-inductance are varied. Hence, the phase between the voltage and current of an inverter output is significantly varied with a coil current variation by inverter control on the other side or a self-output current variation.
In conventional current synchronization control, since position control is performed on the gate pulse of an inverter to perform current synchronization control, control needs to be significantly performed on an inverter voltage position (=pulse position) so that current synchronization is performed. Since a pulse movement range for the current synchronization control is large, it is disadvantageously impossible to stably provide a rapid response in the current synchronization control, and it is disadvantageously impossible to stably increase the speed of the inverter control.
Even when the current synchronization is performed, the mutual induction voltage of an reactive part is high, the inverter needs to overcome this voltage to produce an output voltage, and since an output phase angle is large at this time and a power factor is poor, an inverter converter capacity disadvantageously needs to be increased. In patent document 2, it is proposed that in order to solve this problem, the mutual induction of the coil and a reverse-polarity inductance are provided between the heating coil and the inverter to improve the power factor.
Moreover, even in this state, the inverter output phase is varied by the current variation on the self-side or the other side. When the mutual induction voltage of the reactive part is high, that is, the reactance component of the mutual induction impedance is large, the inverter output phase reaches about 90 degrees or 90 degrees or more, disadvantageously, a switching loss is increased or reverse power is produced to cause a dangerous operation. When the mutual induction voltage of the effective part is high, that is, the resistance component of the mutual induction impedance is large, the inverter output phase reaches 0 degrees or 0 degrees or less, it is disadvantageously impossible to perform a ZVS (zero voltage switching) operation to increase the switching loss or cause a dangerous operation.
Although the above description has been given using an example of a voltage-type inverter (series resonance), the same problem is present even in a current-type inverter (voltage-type inverter).